Method and device for machining a workpiece

ABSTRACT

In a method for machining a workpiece, a laser beam ( 5 ) is guided across the workpiece surface by means of a beam guide ( 2, 51 ), wherein the laser beam guide comprises a first guide ( 2 ) effecting a laser beam guide at a first path speed. The laser beam guide comprises a second guide ( 51 ) simultaneously operating with the first guide, which effects a laser beam guide at a second path speed, which is higher than the first path speed.

The invention relates to a method and a device for machining aworkpiece, in particular for machining a workpiece by means of apulsating laser beam. FIG. 1 shows known features for machining aworkpiece by means of a laser beam.

FIG. 1 a schematically shows a side view of a laser machining device. 1is the workpiece. 4 represents the workpiece table on which theworkpiece 1 is mounted. 5 symbolizes the machining laser beam which isguided in a certain area by a beam guide 2. 3 is the laser light sourcewhich emits laser light, the laser light then travelling into the beamguide 2 and being directed onto the workpiece 1 by it. 6 symbolizes acontrol which can receive process data and which drives at least thebeam guide 2 and the laser source 3. It can be connected to ahigher-ranking control 7. Depending on the design, it can even performhigh-ranking control tasks and can be connected to a memory 7 whichholds cavity data or production data, for example, in compliance withwhich the workpiece 1 is to be machined.

A coordinate system is schematically outlined: the x-direction extendshorizontally in the plane of projection, the z-direction perpendicularlyin the plane of projection and the y-direction perpendicularly to theplane of projection and rearwards.

The device of FIG. 1 a can be used for both forming a cavity by laminarmaterial removal in layers and machining a surface where it is notprimarily the voluminous removal that is of interest but predominantlythe creation of certain surface properties. This can be achieved withinsome few cycles (layers). In an extreme case, it is only necessary totraverse the respective surface points to be processed once (in thex/y-directions).

FIGS. 1 b and 1 c show the conditions when a cavity is formed, i.e. withvoluminous material removal. FIG. 1 b is a plan view of the workpiece,FIG. 1 c is a section through the workpiece. The meandering curve 8 arepresents the movement of the laser beam on the workpiece surface. Inthis connection, it is pointed out that the laser beam has a certaindiameter on the workpiece surface and the width (perpendicularly to thedirection of movement in the plane of the workpiece surface) of impacton the workpiece surface also correlates with this diameter. Typicalbeam diameters range between 5 and 50 μm. In this connection, thedistances of the individual meanders are correspondingly correlatedrelative to one another. Instead of a reciprocating meander, it is alsopossible to use e.g. a spiral or the line-by-line rectified traverse ofmachining lines. Reference numeral 9 a refers to the machining limits inthe respective layer. The laser beam 5 is activated inside the machininglimits and it is deactivated outside of it so that its track only existstheoretically in the areas located outside. The laser beam effects aremoval inside the limits 9 a, but not outside thereof. Instead of thedeactivation outside the machining limits, the selection of a strongdefocusing is also possible so that the energy supply per unit area isno longer sufficient to machine the workpiece and the lasersimultaneously operates continuously. According to FIG. 1 b, a layer isremoved. This is then followed by another layer where differentmachining limits 9 a may apply. In this way, the cavity is formed intothe depth as shown in FIG. 1 c. 1 a defines the momentary cavity bottom,i.e. the momentary workpiece surface, as viewed in FIG. 1 b. Thehorizontally dashed lines schematically show the individual removedlayers. The machining limits 9 a differ from layer to layer, i.e. in thedepth direction (negative z-direction), so as to form in this way acavity having an accurately defined shape. It may concern e.g. the moulddesign or the like. 1 c symbolizes the bottom of the finished cavity. Itis not yet manufactured, but will be exposed in the course of time bythe procedure of removing material in layers and area-wide.

FIG. 1 d shows a procedure of machining a surface. It is a plan view ofthe workpiece surface. Here, one or several individual tracks can beused. Machining may be, but does not have to be, carried out over theentire area. Parallel tracks 8 b, 8 c and 8 d, 8 e may be provided. Inthis way, it is possible to produce a surface having certain optical ormechanical properties. As in FIG. 1 b, FIG. 1 d introduces therespective track by the beam guide 2 of FIG. 1 a. In this connection,the laser beam travels back a certain distance Δx over the workpiecesurface within a certain time At so that correspondingly a path speed vBof the laser beam on the workpiece surface can be defined as Δx/Δt.Common path speeds are 100-300 mm/s. They are limited upwards by theadjusting accuracy decreasing with increasing speed.

FIG. 1 e shows the characteristic of the machining laser beam over time.The particular light output is shown. When the cross-sectional area ofthe laser beam is constant, the temporal course thus also corresponds tothe energy supply based on the unit area. The laser beams preferablypulsate with constant period. The pulse peaks at the point in time t1,t2, t3 are given for which a constant time difference tL is assumed.This corresponds to a laser pulse frequency fL. The following applies:

tL=1/fL.

Typical laser pulse frequencies fL are between 10 kHz and 100 kHz.However, a trend also towards markedly higher pulse frequencies fL hasrecently been found. Machining lasers having pulse frequencies around orabove 1 MHz or around 2 MHz, also above 5 MHz up to 100 MHz or more, aremeanwhile offered. It is desirable to use said lasers since in spite ofthe higher pulse frequency they have a pulse peak output relevantlydetermining the removal and corresponding to that of lasers having lowerfrequencies or even strongly exceeding it. A sample calculation shallfollow for the purpose of clarification: A conventional average laseroutput of 10 W results in a pulse energy of 125 μJ when the pulsefrequency is 80 kHz. With a pulse duration of 100 ns this yields a pulsepeak output of 1250 W.

However, in new systems, an average laser output of 10 W can be suppliedin pulsed fashion with 1 MHz. Here, the pulse durations are markedlyshorter, e.g. 10 ps. This results in a pulse peak output of 1 MW.

FIG. 1 f shows another time diagram. It describes a special case ofmachining a workpiece by means of a pulsating laser beam where a warmingpulse precedes the actual working pulse. In this case, the pulsefrequency to be considered has to be measured between the workingpulses, as shown in FIG. 1 f.

The beam guide 2 has a certain finite adjusting speed. While it can beoperated very rapidly in theory, this, however, affects the adjustingaccuracy and thus the accuracy of machining the surface or forming thecavity on the workpiece. This leads to conditions which are explainedwith reference to FIG. 1 d: FIG. 1 d is a plan view of the workpiecesurface. 8 f refers to the momentary path of the laser beam over theworkpiece surface. Depending on the path speed vB of the laser beam onthe workpiece surface, the beam diameter dS of the laser beam at theworkpiece surface level and the laser pulse frequency fL, the pulses mayoverlap. For example, assuming a pulse frequency fL of 100 kHz and abeam diameter dS of 10 μm, a path speed vB of 1 m/s results incompliance with

vB=dS/tL=dS×fL,

if it is desired that laser light impingement points do not at alloverlap. However, today's beam guides, which are usually composed ofmovable mirrors, are limited as regards their guide speed on theworkpiece surface to about 500 mm/s or 1000 mm/s. This means that theassumed numerical values already result in an overlap of about 50% ofthe beam diameter dS. If it is now assumed that an incoming laser pulseheats the material so as to liquefy and evaporate it (wherein the liquidphase can be passed through very rapidly), the result thereof in anycase is that the subsequent laser pulse impinges on a point where theprevious laser pulse was also effective immediately before.

This leads to various unpleasant effects. As a result, the workpiece isheated by the introduction of excess energy per unit time into the sameabsorption area which was hit earlier by previous pulses, also farbeyond the laser beam limit, i.e. beyond the beam diameter, and isliquefied in a larger area, which results in an undesired increase inthe melting portion. This in turn leads to the circumstance that theconditions are no longer accurately defined as far as the interactionbetween laser beam and workpiece surface is concerned. The removal ofmaterial thus becomes non-uniform and not accurately predictable.

The problem gets worse with the initially mentioned increasing pulsefrequencies. The above indicated numerical example shows that overlapsoccur with already comparatively low pulse frequencies (100 kHz) in thelight of the adjusting speeds of the laser beam guide 2. This will beall the more the case if the pulse frequency is not 100 kHz, forexample, but rather 1 MHz or more and up to 100 MHz, for example. In acertain way, it is then possible to try to reduce the laser beamdiameter or improve the adjusting speed. However, a significant overlapof individual pulses on the workpiece surface cannot be avoided withtoday's technology, as shown in FIG. 1 g. In the case of high pulsefrequencies of 500 kHz and more, it has to be assumed that with theprior art technology the overlap would be greater than 50% of the beamdiameter on the workpiece surface and probably also greater than 80% ofthe beam diameter. This results even more in said inaccuracies withrespect to the removal output.

Further prior art is found in DE102005039833A1, DE102004051180A1,DE10392185T5, EP0536625B1, DE10309157A1 and U.S. Pat. No. 5,837,962.

It is the object of this invention to provide a method and a device formachining a workpiece, which enable a stable and predictable control andadjustment of the removal output even with a high pulse frequency.

According to an embodiment of the invention, the path speed vB at whichthe laser beam is guided over the workpiece surface is adjusted bysuitable technical means so as to yield the following conditions in thelight of laser pulse frequency fL and beam diameter dS:

vB>n×dS×fL,

wherein n is a proportional factor which can be 0.5 or 0.7 or 1 orgreater than 1. If the path speed changes over time, an average pathspeed or the maximum path speed can be used. The same applies to thelaser pulse frequency fL.

For example, a fast path speed vB can be suitably adjusted such that afaster further movement having an optionally smaller amplitude issuperimposed on the movement effected by the prior art beam guide 2.Then, two guides are provided one of which is the conventional beamguide, as described with reference to FIG. 1 a, and the other of whichis another beam guide which produces the superimposed guide of the laserbeam.

A device for machining a workpiece can be designed for carrying out oneof the above described methods and/or the variants described in thisconnection and those to be described below.

The workpiece to be machined can be or comprise a metallic material or asemiconductor material or ceramics or glass or plastics. The machiningcan be surface machining to influence the optical appearance or theroughness of a surface or it can be the formation of a cavity such thatmaterial is removed in layers and in laminar fashion so as to create acavity having accurately defined side walls and an accurately definedbottom. When the cavity is formed, the accuracies may be better than aproduction accuracy of 100 μm, preferably better than 50 or 10 μm.

Each of the pulses can have sufficient energy for melting or evaporatingthe material at the point of impingement. In the embodiment of FIG. 1 f,this applies to the actual working pulses and not necessarily to thepreceding warming pulses.

Embodiments of the invention are described below with reference to thedrawings, wherein

FIG. 1 shows prior art illustrations,

FIG. 2 shows an illustration explaining the effect of the invention,

FIG. 3 shows embodiments according to the invention,

FIG. 4 shows another embodiment according to the invention, and

FIG. 5 shows a device according to the invention.

FIG. 2 shows the conditions according to the invention. Referencenumeral 8 g shows the momentary path of the laser beam. It may becurved. 21 and 22 are two successive points of impingement of twosuccessive working pulses of the laser light. Their imaginary centersare points 21 a and 22 a, respectively. Circles 21 b and 22 b designatein each case the diameter of the laser light beam. The distance wLbetween both centers 21 a and 22 a is the distance the laser beamtravels between two working pulses. The conditions are adjusted suchthat the overlap is smaller than a predetermined value, in particularsmaller than 30% or 50%. In general, the conditions can be described bythe formula

vB>n*dS*fL

wherein vB is the path speed of the pulsating laser beam on theworkpiece surface, dS is the beam diameter at the workpiece surfacelevel and fL is the pulse frequency of the working pulses of thepulsating laser light. In this connection, the pulse frequency can begreater than 100 kHz, preferably greater than 1 MHz and more preferablygreater than 10 MHz. The value n is a proportional factor which may be0.3 or 0.5 or also 1 or greater than 1. The overlap (corresponding tothe proportional factor n) can be adjusted in accordance with thedesired conditions.

The path speed is determined to be the speed resulting when the pathcovered between two pulses (wL) is divided by the time required for thedistance (corresponding to tL=1/fL). In the case of variable speeds, itis possible to use an average value or in each case a momentary valuetogether with momentary values of the respectively other variables.

The required path speed can be produced by generating a guide movementsuperimposed on the conventional guide. On the one hand, the laser beamcan be guided by a conventional beam guide and, on the other hand, byanother beam guide which generates a faster movement. Here, theconventional beam guide (reference numeral 2 in FIG. 1 a) can have anamplitude (maximum deflection) greater than that of the beam guide addedaccording to the invention. The added beam guide can generate adeflection which has a component that is transverse to the movementgenerated by the conventional beam guide 2. The additional guide can usea reciprocating movement or also a superimposed movement in bothdimensions of the workpiece surface. Its amplitude can be smaller thanthat of the maximum deflection of the conventional beam guide. As far asa reciprocating movement or a movement periodic in some respects isgenerated by the added beam guide, its frequency can be a fraction ofthe frequency of the laser working pulses, e.g. more than 5% or morethan 10% or 1/n (n=integer) of the laser pulse frequency. Severalworking pulses of the laser beam are produced per period of theadditional movement.

FIG. 3 shows how to generate the desired movement in variousembodiments. The movement patterns of FIG. 3 are presentations of laserbeam guides in the workpiece surface plane, i.e. in the x/y planes.

FIGS. 3 a, 3 b and 3 c show beam guides as can be generated by theconventional beam guide. In this context, FIG. 3 a corresponds, at leastarea by area, to the beam guide of FIG. 1 d, and FIG. 3 b corresponds tothe beam guide of FIG. 1 b. FIG. 3 c shows two defined paths 8 h and 8 iwhich are successively adjusted by the conventional beam guide and whichmay also intersect.

FIGS. 3 d, 3 e and 3 f show movement patterns which can be adjusted byanother beam guide and which can be superimposed on the movementgenerated by the conventional beam guide. Pattern 31 shows a verticalup-and-down movement during which several laser pulses may occur in acycle. For example, if during a period of the movement of FIG. 3 d ten(general: n) working pulses are to occur, the periodicity of themovement will be one tenth (in general: 1/n) of the laser pulsefrequency. The deflection 2 a (double amplitude A) can be selected inaccordance with the beam diameter dS. It may be a multiple of the laserbeam diameter, e.g. more than 2-fold, more than 10-fold or more than50-fold. On the other hand, the deflection 2 a can be less than tentimes or also less than five times the laser beam diameter dS at theworkpiece level. In general, the technically possible deflection rangeof the second guide can be utilized. With +/−1° and a path length of 200mm, for example, a deflection of almost 7 mm results. In order to avoidboundary effects, extremes in the use can be avoided.

FIG. 3 e shows another pattern where a circular motion is introducedwhich is superimposed on the conventional movement. Several machiningpulses occur during a cycle. Considerations on the periodicity are thesame as those with respect to FIG. 3 d.

The movement of FIG. 3 d can be changed so as to change the oscillationdirection. For example, it may be such that there remains at leastalways one certain minimum angle (e.g. 20° or 40°) with respect to thedirection of movement which is introduced by the conventional beamguide. For example, it is possible to switch between a verticaloscillation (FIG. 3 d) and a horizontal oscillation turned by 90° withrespect thereto. FIG. 3 f shows an embodiment where the superimposedbeam guide is random or quasi-random. It can have movement components inthe guide direction of the conventional beam guide and perpendicularlythereto. For example, in a rectangular or square grid pattern, theindividual positions can be driven quasi-randomly, yet in apredetermined way, until all positions of the grid pattern have beenreached once. Then, the pattern is repeated.

FIG. 4 shows the result of the superimposition of e.g. the conventionalguide in accordance with a line of FIG. 3 a (e.g. line 8 b), forexample, and the circular additional guide of FIG. 3 e. A skewed spiralresults. 23 marks the path along which the laser beam is guided onaccount of the superimposed movements. The fat dots 41, 42, 43, 44, 45,. . . mark the centers of the impingement positions of the individualworking pulses of the laser light. In addition to the centers 41 and 42,the respective laser beam diameters 41 a, 42 a are also drawn. Thedrawing is such that the individual impingement points of successivepulses just abut. However, the dimensions can also be such that theyoverlap more strongly or are farther apart from one another than shownin the drawing. With a constant working laser pulse frequency fL andgiven conventional guide this can be achieved, on the one hand, by arotational frequency control with respect to the laser pulse frequencyfL and, on the other hand, by the amplitude control of the superimposedmovement.

As regards its own path speed, the superimposed movement is usuallymarkedly higher than that of the movement driven by the conventionalguide. However, the amplitude (maximum possible deflection) is alsousually smaller. The superimposed movement can be such that a certainpoint on the workpiece is hit twice or several times by a laser beamwhen it travels over the area. On the other hand, the parameters canalso be adjusted so as to also achieve a reliable area-wide and onlysingle traverse over all points of the workpiece surface to be machinedwith the superimposed movement when the surface is skimmed. For example,this can be effected by suitably selecting the deflection amplitude,step width and frequency of the superimposed movement.

In general, the result of the superimposed guide can be that from amacroscopic point of view the guide is similar to the conventional guidein so far as the large movement over the workpiece is predetermined bythe conventional guide as usual. However, the traveled paths can bewider due to the additional lateral deflection so as to have widertracks from many more or less adjoining operating points as compared tothe prior art technology. Correspondingly, the conventional guide canintroduce tracks which are spaced farther apart.

FIG. 5 schematically shows a machining device according to theinvention. The same reference numerals as those in FIG. 1 a show equalcomponents. 5 a and 5 b designate in dashed lines the maximum possibledeflections of the laser beam 5 which can be caused by the beam guide 2.Correspondingly, they define a working window on the workpiece surface.

A further beam guide 51 is provided which may be within the beam pathupstream of the conventional beam guide 2. It also generates an angulardeflection of the laser beam, whose maximum limitation is shown bydashed lines 5 c and 5 d. According to the smaller amplitude of theadditional beam guide device 51, the maximum possible angle of movementcan be smaller than that of the conventional beam guide 2. However, theadditional beam guide 51 can also be provided uniformly with theconventional beam guide 2, e.g. by adjusting mirrors of the conventionalbeam guide apparatus in themselves once again or the like. Theadditional second guide device 51 can also be driven by the control 6.Depending on the requirement, it can be activated and deactivatedseparately so that it can be added to the conventional guide, whereappropriate.

The drive signals to the second guide device 51 are generated so as toform the respectively desired guide patterns, in particular one of thoseof FIG. 3 d, 3 e or 3 f. The second guide can effect a deflection of thelaser beam in both dimensions of the surface of the workpiece (x, y).However, in another embodiment only a one-dimensional deflection can beadjustable.

The second guide 51 can have one or several acousto-optical elements orelectro-optical elements which determine the deflection according to thefrequency of an applied acoustic or electrical signal, for example. Itcan also have one or several piezoelectric elements. It may also be amechanically oscillating element, e.g. a rotary transparent orreflecting disk located in the beam path and having different directionproperties. In order to effect a deflection along two dimensions(corresponding to x and y on the workpiece surface), it is possible toprovide, where appropriate, elements intersecting in their effectivedirection.

When the second guide operates periodically, its working period can beadjusted to the laser pulse frequency fL, e.g. such that the workingfrequency of the second guide is a preferably integer fraction of thelaser pulse frequency fL. On the other hand, an accurate adjustment ofthe frequencies and/or period durations can also be provided such thatthe following applies to the period duration ‘T2 of the second guide:

1/f2=T2=n*tL+tR

wherein n is the number of working pulses per period of the second guideand tR is a duration required for the return of the second guide andother time-outs.

The guide direction of the second guide can be permanent in onedirection (reciprocating). However, this one direction can be variablein time. Yet, the working laser beam can also be deflectedsystematically such that it is deflected on the workpiece surface in thetwo dimensions thereof (x, y). The deflection can be adjusted such thatit always has a certain angular relationship with respect to themomentary direction of movement which is generated by the first guide bybeing aligned with respect thereto or by being rectangular thereto orobserving a certain angular range.

The invention is suitable for both the machining of a surface and theformation of a cavity. When the surface is machined, a single skimmingof all surface areas to be machined can be sufficient. When the cavityis formed, certain areas are usually skimmed in the x-y plane severaltimes, material having a certain thickness being removed each time. Dueto the plurality of removed layers one then protrudes deeper and deeperinto the cavity.

In this connection, the layer thicknesses or penetration depths perpulse may be 10 preferably below 2 or 1 μm, more preferably below 0.25μM.

The lower limit of the path speed caused by the first guide alone can be20 or 50 or 100 mm/s. Its upper limit can be 100 mm/s or 200 mm/s or 500mm/s.

Considered laser beam diameters at the level of the workpiece surfacecan have 50 μm or 20 μm as the upper limit and 2 μm or 5 μm or 10 μm asthe lower limit.

The amplitude of the deflection caused by the second guide can belimited upwards to 2 mm or to 1 mm or to 50 μm. It can also be limitedupwards to 200 times or 100 times or 50 times the diameter of the laserbeam on the workpiece surface. The amplitude can be limited downwards to5 μm or 10 μm or also to two times the diameter of the laser beam on theworkpiece surface.

The frequency f2 of the second guide (reciprocal value of the periodduration t2) can be greater than n times the laser period tL, n being 3or 5 or 7.

When the beam guide is introduced by the second guide, the workingpositions can also be tracked to the effect that the actual workingpositions of the laser beam (caused by the effects of the first and thesecond guides) are tracked and recorded and that according to theserecords further drives of the second guide are made, in particular to“hit” areas that have not been hit so far. Correspondingly, a detectiondevice for the already machined positions on the workpiece surface canbe provided as well as a memory to store the result of the detection andprovide it for the subsequent evaluation. The diameter of the laser beamon the workpiece surface can be determined by focusing on an intensitylimit value for determining the diameter as compared to the centralintensity when the cross-section is not clearly defined, e.g. to 50% orto a value 1/e or 1/e² of the central intensity. ISO 11146 can beconsidered.

1. A method for machining a workpiece wherein a laser beam pulsatingwith a pulse frequency fL is guided over the workpiece surface by meansof a beam guide, characterized in that the laser beam is guided suchthat its path speed vB on the workpiece surface complies with thefollowing condition:vB>n*dS*fL wherein n is a proportional factor which can be 0.2 or 0.4 or0.6 or 0.8 or 1 or greater, dS is the beam diameter at the workpiecelevel and wherein the pulse frequency is greater than 100 kHz.
 2. Themethod for machining a workpiece, in particular according to claim 1,wherein a laser beam is guided by means of a beam guide over theworkpiece surface, the laser beam guide comprising a first guide whichon its own effects a laser beam guide with a first path speed,characterized in that the laser beam guide comprises a second guidesimultaneously operating with the first guide, which on its own effectsa laser beam guide with a second path speed which is higher than thefirst path speed.
 3. The method according to claim 2, characterized inthat the amplitude of the first guide is higher than that of the secondguide.
 4. The method according to claim 2, characterized in that thefirst guide and/or the second guide effect a laser beam movement alongtwo dimensions on the workpiece surface.
 5. The method according toclaim 2, characterized in that when viewed in laser light propagationdirection initially the second guide and then the first guide influencethe laser beam.
 6. The method according to claim 2, characterized inthat the second guide is electro-optical, acousto-optical orpiezoelectric.
 7. The method according to claim 2, characterized in thatthe second guide guides the laser beam in randomly determined fashion.8. The method according to claim 1, characterized in that the secondguide guides the laser beam in a direction which differs from the guidedirection of the first guide.
 9. The method according to claim 2,characterized in that the first and second guides are superimposed. 10.The method according to claim 1, which is designed for machining asurface or for forming a cavity.
 11. The method according to claim 1,characterized in that each pulse of the pulsating laser beam is suitablefor removing material.
 12. A device for machining a workpiece, inparticular for carrying out the method according to claim 1, comprisinga laser light source for generating a pulsating laser beam, and a beamguide for guiding the laser beam over the workpiece surface,characterized in that the laser light source generates the laser beamwith a pulse frequency of at least 100 kHz, and the beam guide guidesthe laser beam such that its path speed vB on the workpiece surfacecomplies with the following condition:vB>n*dS*fL wherein n is a proportional factor which can be 0.2 or 0.4 or0.6 or 0.8 or 1 or greater, dS is the beam diameter at the workpiecelevel and fL is the pulse frequency of the working pulses of the laserbeam.
 13. The device, in particular according to claim 12, for machininga workpiece, comprising a laser light source for generating a pulsatinglaser beam, and a beam guide for guiding the laser beam over theworkpiece surface, which has a first guide means which effects a laserbeam guide with a first path speed, characterized in that the beam guidehas a second guide means simultaneously operable with the first guidedevice, which effects a laser beam guide with a second path speed whichis higher than the first path speed.
 14. The device according to claim13, characterized in that the second guide means comprises one or moreof the following elements: an electro-optical element, anacousto-optical element, a translationally or rotatorily adjustableoptical element.
 15. The device according to claim 13, characterized inthat the first guide means is in the beam path downstream of the secondguide means.